1. Field of the Invention
The present invention relates to a developing method for an electrostatic latent image which is formed in xerography, electrostatic printing, electrographic recording using a one-component developer.
2. Description of the Prior Art
Generally, wet development using a liquid developer and dry development using a powder developer are known and have previously been used to form a visual image after developing an electrostatic latent image. The dry development is advantageous in that it uses a plain paper, and the process is classified into two major methods: a method using a two-component developer consisting essentially of carrier and toner, and a method using a one-component developer consisting essentially of toner.
The developing method using a one-component developer is superior to the method using a two-component developer in some respects. Specifically, the method does not intrinsically produce changes in the density of toner, thereby enabling the structure of a developing machine to be kept simple, and the characteristics of the developer are stable over a long term without deterioration. However, this method is quite disadvantageous in that it cannot form images stably, because it is difficult to bring a one-component developer into any desired charged state, and because it is difficult to use such a developer on a surface for an electrostatic latent image in its desired state upon developing.
In order to have a good visual image using dry development, it is necessary to charge toner so that it has an appropriate quantity of charge which is opposite in polarity to an electrostatically charged image to be developed. The method using a two-component developer mechanically stirs toner and carrier to frictionally electrify the toner, so that the polarity and quantity of charge of the toner can be controlled to a considerable extent by selecting characteristics of the carrier, conditions of stirring, etc. However, in the method using a one-component developer consisting only of toner, there exists no such carrier, thus rendering control of polarity and quantity of charge of toner very difficult.
For charging a one-component developer, frictional charging using a mechanical force, charge injection electrification using an electrode that is injected and charging using a corona discharger are known.
The method using frictional charging employs friction between toner and any one of a stirrer, a conveyance member or the wall of a container of friction among particles of toner to charge the toner. The result is that the quantity of charge is generally limited and its magnitude cannot be controlled with ease. Further disadvantageously, as a consequence of such process, a portion of toner may be electrified to have a polarity opposite to the necessary polarity. A method using electric and magnetic forces to move toner on a charging member is capable of charging the toner considerably, but it is difficult for this method to totally remove toner having the opposite polarity.
Charge injection electrification has the disadvantage that if the developer is insulative, then it is difficult to inject charge, while if it is electrically conductive, leak tends to occur. Therefore, it is impossible to obtain a large quantity of charge.
Electrification using a corona discharger is disadvantageous in that it is unable to electrify developer uniformly.
Various means (referred to "developing means" hereinafter) for causing a charged one-component developer to work on a charge receptor which forms an electrostatic latent image are known, and they are classified by the following methods: a contacting development method, such as an impression method, in which developer is brought into contact with the total surface of the charge receptor; a non-contacting development method in which an electric field causes toner to adhere without bringing developer into contact with the total surface of the charge receptor, such as described in U.S. Pat. No. 3,866,574, or Japanese Patent Laid-Open to Public Inspection Nos. 18656/1980 through 18659/1980, for example, or by a touchdown method.
Generally, it is necessary for development using a one-component developer to move the developer, that is toner, to a developing station, while it is unipolarly charged, whether the method is a contact development or noncontacting development. Development using bipolarly charged toner cannot obtain a high-quality visual image, because toner of the opposite polarity adheres to the edge of an image, thus to deteriorate the quality of the image, and because toner of opposite polarity adheres to the background resulting in fog.
Particularly, the noncontacting development requires that toner is unipolarly charged, and that a thin layer (thickness equivalent to several particles of toner) is formed quite uniformly on a developing sleeve. If the thickness of toner is too great, then overdevelopment will occur, and massed particles of toner will participate in development without being separately emitted, resulting in deterioration in image quality. Further disadvantageously, toner is emitted excessively.